But is it guaranteed that re.compile will always yield an object of the same type?
Guido> There are no guarantees in life, but I expect that that is something Guido> that plenty of code depends on, so it will likely stay that way. The kind of situation I imagine is that a regular expression might be implemented not just as a single type but as a whole hierarchy of them, with the particular type used for a regular expression depending on thevalue of the regular expression. For example: class Compiled_regexp(object): # ... class Anchored_regexp(Compiled_regexp): # ... class Unanchored_regexp(Compiled_regexp): # ... where whether a regexp is anchored or unanchored depends on whether it begins with "^". (Contrived, but you get the idea). In that case, it is entirely possible that re.compile("") and re.compile("^foo") return types such that neither is an instance of the other. I understand that the regexp library doesn't work this way, and will probably never work this way, but I'm using this example to show why the technique of using the type returned by a particular library function call to identify the results of future calls doesn't work in general.